DE69025548T2 - Absorbent article made of fibers cross-linked by polycarboxylic acids - Google Patents

Abstract

Disclosed are absorbent structures containing individualized, crosslinked fibers. The individualized, crosslinked fibers preferably have a C2 - C9 polycarboxylic acid crosslinking agent reacted with the fibers in the form of intrafiber crosslink bonds. Preferably, the crosslinking agent is citric acid, and between about 0.5 mole % and about 10.0 mole % crosslinking agent react to form the intrafiber crosslink bonds. Also preferably, the absorbent structures have actual dry densities greater than their corresponding equilibrium wet densities, and expand upon wetting. The absorbent structures may also contain hydrogel-forming material.

Description

FIELD OF INVENTION

This invention relates to cellulosic fibers having high fluid absorption properties, and more particularly to absorbent structures made from such cellulosic fibers. In particular, this invention relates to absorbent structures containing individualized, cross-linked cellulosic fibers.

BACKGROUND OF THE INVENTION

Fibers that are crosslinked in a substantially individualized form, as well as various processes for making such fibers, are described in the literature. The term "individualized, crosslinked fibers" refers to cellulosic fibers that have primarily intrafiber chemical crosslinking bonds. That is, the crosslinking bonds are primarily between cellulose molecules of a single fiber rather than between cellulose molecules of different fibers. Individualized, crosslinked fibers are generally considered useful in absorbent product applications. The fibers themselves, as well as absorbent structures containing individualized, crosslinked fibers, generally exhibit an improvement in at least one significant absorbent property relative to conventional, uncrosslinked fibers. Often, the improvement in absorbency is noted with reference to absorbent capacity. In addition, absorbent structures made from individualized, crosslinked fibers typically exhibit increased wet resiliency and increased dry resiliency relative to absorbent structures made from uncrosslinked fibers. The term "elasticity" shall refer to the ability of cushions made from cellulose-based fibers to return to an expanded original state after a compression force is released. Dry elasticity refers particularly to the ability of an absorbent structure to expand upon release of a compressive force applied while the fibers are in a substantially dry state. Wet strength particularly to the ability of an absorbent structure to expand upon release of a compressive force applied while the fibers are in a moistened state. For the purposes of this invention and the consequentiality of the disclosure, wet resilience shall be observed and recorded for an absorbent structure moistened to saturation.

In general, three categories of processes have been reported to produce individualized crosslinked fibers. These processes, described below, are referred to herein as dry crosslinking processes, aqueous solution crosslinking processes, and substantially non-aqueous solution crosslinking processes.

Methods for making individualized, crosslinked fibers using dry crosslinking technology are described in U.S. Patent 3,224,926, L.J. Bernardin, issued December 21, 1965. Individualized, crosslinked fibers are made by impregnating swollen fibers with crosslinking agent in an aqueous solution, dewatering and defibrating the fibers by mechanical action, and drying the fibers at elevated temperature to effect crosslinking while the fibers are in a substantially individual state. The fibers are inherently crosslinked in an unswollen, collapsed state as a result of being dehydrated prior to crosslinking. Methods such as those described in U.S. Patent 3,224,926, in which crosslinking is induced while the fibers are in an unswollen, collapsed state are referred to as methods for making "dry crosslinked" fibers. Dry crosslinked fibers are generally highly stiffened by crosslinking bonds and absorbent structures made from these fibers exhibit relatively high wet and dry elasticity. Dry crosslinked Fibers are also characterized by low fluid retention values (FRV).

Methods for making aqueous solution crosslinked fibers are disclosed, for example, in U.S. Patent 3,241,553, F.H. Steiger, issued March 22, 1966. Individualized crosslinked fibers are made by crosslinking the fibers in an aqueous solution containing a crosslinking agent and a catalyst. Fibers made in this way are hereinafter referred to as "aqueous solution crosslinked" fibers. Due to the swelling action of water on cellulosic fibers, aqueous solution crosslinked fibers are crosslinked while in an uncollapsed, swollen state. Relative to dry-crosslinked fibers, fibers crosslinked in aqueous solution, as disclosed in U.S. Patent 3,241,553, have greater flexibility and lower stiffness and are characterized by higher fluid retention values (FRV). Absorbent structures made from aqueous-solution crosslinked fibers have lower wet and dry resilience than structures made from dry-crosslinked fibers.

In U.S. Patent 4,035,147, Sangenis et al., issued July 12, 1977, a process for making individualized crosslinked fibers is disclosed in which dehydrated, unswollen fibers are contacted with crosslinking agent and catalyst in a substantially non-aqueous solution containing an amount of water insufficient to cause the fibers to swell. Crosslinking occurs while the fibers are in this substantially non-aqueous solution. This type of process shall hereinafter be referred to as a non-aqueous solution crosslinking process; and the fibers produced thereby shall be referred to as non-aqueous solution crosslinked fibers. The non-aqueous solution crosslinked fibers disclosed in U.S. Patent 4,035,147 do not swell upon prolonged contact with solutions known to those skilled in the art as swelling agents. Just like dry cross-linked fibers, they are strongly stiffened by cross-linking bonds and absorbent structures that made from it have relatively high wet and dry elasticity.

Crosslinked fibers as described above are believed to be suitable for applications in lower density absorbent products such as diapers as well as for applications in higher density absorbent products such as catamenials. However, such fibers have not provided sufficient absorbency benefits in comparison to their disadvantages and costs compared to conventional fibers to result in significant commercial success. The commercial appeal of crosslinked fibers has also suffered from safety concerns. The most commonly reported crosslinking agents in the literature are formaldehyde and formaldehyde addition products known as N-methylol agents or N-methylolamides, which unfortunately cause irritation to human skin and are associated with other human safety concerns. Separation of free formaldehyde to sufficiently low levels in the crosslinked product so as to avoid concerns about skin irritation or other human safety concerns has been prevented by both technical and economic limitations.

As mentioned above, the use of formaldehyde and various formaldehyde addition products to crosslink cellulosic fibers is well known in the art. See, for example, U.S. Patent 3,224,926, Bernardin, issued December 21, 1965; U.S. Patent 3,241,553, Steiger, issued March 22, 1966; U.S. Patent 3,932,209, Chatterjee, issued January 13, 1976; U.S. Patent 4,035,147, Sangenis et al., issued July 12, 1977; and U.S. Patent 3,756,913, Wodka, issued September 4, 1973. Unfortunately, the irritating effect of formaldehyde vapor on the eyes and skin is a significant disadvantage of such processes. There is therefore a clear need for cross-linking agents for cellulose-based fibres that do not require formaldehyde or its unstable derivatives.

Other references disclose the use of dialdehyde crosslinking agents. See, for example, U.S. Patent 4,689,118, Makoui et al., issued August 25, 1987; and U.S. Patent 4,822,453, Dean et al., issued April 18, 1989. The Dean et al. reference discloses absorbent structures containing individualized, crosslinked fibers, wherein the crosslinking agent is selected from the group consisting of C2 -C8 dialdehydes, with glutaraldehyde being preferred. According to these references, many of the disadvantages associated with formaldehyde and/or formaldehyde addition products appear to be overcome. However, the costs associated with producing fibers crosslinked with dialdehyde crosslinking agents such as glutaraldehyde may be too high to result in significant commercial success. Therefore, there is a need for crosslinking agents for cellulosic fibers that are both safe for use on human skin and commercially viable.

The use of polycarboxylic acids to impart wrinkle resistance to cotton fabrics is well known in the art. See, for example, U.S. Patent 3,526,048, Roland et al., issued September 1, 1970; U.S. Patent 2,971,815, Bullock et al., issued February 14, 1961; and U.S. Patent 4,820,307, Welch et al., issued April 11, 1989. These references all concern the treatment of cotton fabrics with polycarboxylic acids and special curing catalysts to improve the wrinkle resistance and durability properties of the treated fabrics.

It has now been discovered that ester crosslinks can be induced in individualized cellulosic fibers by the use of specific polycarboxylic acid crosslinking agents. The ester crosslinking bonds induced by polycarboxylic acid crosslinking agents differ from crosslinking bonds derived from the mono- and dialdehyde crosslinking agents, which form acetalic crosslinking bonds. Applicants have found that absorbent structures made from these individualized, ester crosslinked fibers exhibit increased wet resilience and dry resilience as well as improved response to wetting relative to structures containing uncrosslinked fibers. Importantly, the polycarboxylic acids disclosed for use in the present invention, in contrast to the formaldehyde and formaldehyde addition products commonly used in the art. In addition, the preferred polycarboxylic acid crosslinking agent, ie citric acid, is available in large quantities at relatively low prices, which allows it to compete commercially with the aldehyde crosslinking agents without any concerns about human safety.

It is an object of this invention to provide individualized fibers crosslinked with a polycarboxylic acid crosslinking agent and absorbent structures made from such fibers, wherein the absorbent structures made from the crosslinked fibers have higher levels of absorbent capacity relative to absorbent structures made from uncrosslinked fibers and exhibit higher wet resilience and higher dry resilience than structures made from uncrosslinked fibers.

It is a further object of this invention to provide individualized fibers crosslinked with a polycarboxylic acid crosslinking agent and absorbent structures made from such fibers as described above which have an improved balance of absorbent properties relative to previously known crosslinked fibers.

It is an additional object of this invention to provide commercially viable individualized cross-linked fibers and absorbent structures made from such fibers as described above that can be used in the vicinity of human skin without safety concerns.

It is a further object of this invention to provide absorbent structures with improved absorbent capacity and improved wicking action which, in actual use, cause high levels of skin dryness of the wearer.

SUMMARY OF THE INVENTION

It was found that the above mentioned objectives can be achieved by individualized, cross-linked fibers and incorporation of these fibers into absorbent structures as disclosed herein. Preferably, the individualized, crosslinked fibers comprise, calculated on a molar basis of cellulose anhydroglucose, from about 0.5 mole percent to about 10.0 mole percent, more preferably between about 1.5 mole percent and about 6.0 mole percent, of crosslinking agent reacted with the fibers in the form of intrafiber crosslink bonds, wherein the crosslinking agent is selected from the group consisting of C2 -C9 polycarboxylic acids. The crosslinking agent is reacted with the fibers in an intrafiber crosslink bond form. Such fibers characterized by water retention values (WRVs) of from about 25 to about 60, more preferably from about 28 to about 50, have been found to achieve the goals stated in connection with individualized, crosslinked fibers and provide unexpectedly high absorbent performance in absorbent structure applications.

Without limiting the scope of the invention, the individualized, cross-linked fibers are preferably formed into compressed absorbent structures that expand when moistened.

The absorbent structures may additionally contain hydrogel-forming material. Significantly improved skin dryness and absorbent capacity and dryness of the user's skin can be achieved by using hydrogel-forming material and individualized, cross-linked fibers together. Significantly improved wicking and absorbent capacity are achieved by using individualized, cross-linked fibers together with hydrogel-forming material compared to using conventional, non-cross-linked cellulosic fibers together with hydrogel-forming material. Surprisingly, such improved results can be achieved by using lower amounts of hydrogel-forming material, calculated on a weight basis, in pads containing individualized, cross-linked fibers compared to pads made from conventional cellulosic fibers.

DETAILED DESCRIPTION OF THE INVENTION

Cellulose-based fibers from various natural sources are usable for the invention. from softwood, hardwood or cotton linters are preferably used. Fibers from esparto grass, bagasse, kemp, flax and other sources of lignin-containing fibers and cellulosic fibers may also be used as raw materials in the invention. The fibers may be supplied in slurry form, in sheet form or not as sheets. Fibers supplied as wet web, dry web or other sheet form are preferably converted to the slurry form by mechanical disruption of the sheet, preferably prior to contact of the fibers with the crosslinking agent. Also preferably, the fibers are provided in a wet or moistened state. Most preferably, the fibers are never dried fibers. In the case of dry web, it is advantageous to moisten the fibers prior to mechanical disruption to minimize damage to the fibers.

The optimum fiber source used in connection with this invention will depend on the particular end use contemplated. In general, wood pulp fibers prepared by chemical pulping processes are preferred. Fully bleached, partially bleached and unbleached fibers are useful. It may often be desirable to use bleached pulp because of its greater brightness and greater consumer acceptance. For products such as paper towels and absorbent pads for diapers, sanitary napkins, catamenials and other similar absorbent paper products, it is particularly preferred to use fibers from southern softwood pulp because of their excellent absorbent properties.

Crosslinking agents useful in the present development include aliphatic and alicyclic C2-C9 polycarboxylic acids. As used herein, the term "C2-C9 polycarboxylic acid" refers to an organic acid containing two or more carboxyl (COOH) groups, having from 2 to 9 carbon atoms in the chain or ring to which the carboxyl groups are attached. The carboxyl groups are not included in determining the number of carbon atoms in the chain or ring. For example, 1,2,3- Propanetricarboxylic acid would be considered a C3 polycarboxylic acid containing three carboxyl groups. Similarly, 1,2,3,4-butanetetracarboxylic acid would be considered a C4 polycarboxylic acid containing four carboxyl groups.

More particularly, the C2-C9 polycarboxylic acids suitable for use as cellulose crosslinking agents in accordance with the present invention include aliphatic and alicyclic acids which are either olefinically saturated or unsaturated and have at least three and preferably more carboxyl groups per molecule or contain two carboxyl groups per molecule when a carbon-carbon double bond is present in the alpha, beta position to one or both carboxyl groups. An additional requirement is that in order to effect reactivity in the esterification of cellulose hydroxyl groups, a given carboxyl group in an aliphatic or alicyclic polycarboxylic acid must be separated from a second carboxyl group by not less than two carbon atoms and not more than three carbon atoms. Without being bound by theory, it appears from these requirements that in order for a carboxyl group to be reactive, it must be capable of forming a cyclic 5- or 6-membered anhydride ring with an adjacent carboxyl group in the polycarboxylic acid molecule. Where two carboxyl groups are separated by a carbon-carbon double bond, or where both are attached to the same ring, the two carboxyl groups must be in the cis configuration relative to each other if they are to interact in this way.

In aliphatic polycarboxylic acids containing three or more carboxyl groups per molecule, a hydroxyl group attached to a carbon atom alpha to a carboxyl group does not interfere with the esterification and cross-linking of the cellulosic fibers by the acid. Thus, polycarboxylic acids such as citric acid (also known as 2-hydroxy-1,2,3-propane tricarboxylic acid) and tartrate monosuccinic acids are suitable as cross-linking agents in the present development.

The crosslinking agents based on aliphatic or alicyclic C2-C9 polycarboxylic acids may also contain oxygen or sulfur atom(s) in the chain or ring to which the carboxyl groups are attached. Thus, polycarboxylic acids such as oxydisuccinic acid, also known as 2,2'-oxybis(butanedioic acid), thiodisuccinic acid, and the like are also intended to be within the scope of the invention. For purposes of the present invention, oxydisuccinic acid would be referred to as a C4 polycarboxylic acid containing four carboxyl groups.

Examples of specific polycarboxylic acids falling within the scope of this invention include the following: maleic acid, citraconic acid, also known as methylmaleic acid, citric acid, itaconic acid, also known as methylenesuccinic acid, tricarballylic acid, also known as 1,2,3-propanetricarboxylic acid, trans-aconitic acid, also known as trans-1-propene-1,2,3-tricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid, all-cis-1,2,3,4-cyclopentanetetracarboxylic acid, mellitic acid, also known as benzenehexacarboxylic acid, and oxydisuccinic acid, also known as 2,2'-oxy-bis-(butanedioic acid). The above list of specific polycarboxylic acids is for illustrative purposes only and is not intended to be all-inclusive. It is essential that the cross-linking agent must be able to react with at least two hydroxyl groups on adjacently located cellulose chains in a single cellulose fiber.

Preferably, the C2-C9 polycarboxylic acids used herein are aliphatic, saturated, and contain at least three carboxyl groups per molecule. One group of preferred polycarboxylic acid crosslinking agents for use in the present invention includes citric acid, also known as 2-hydroxy-1,2,3-propanetricarboxylic acid, 1,2,3-propanetricarboxylic acid, and 1,2,3,4-butanetetracarboxylic acid. Citric acid is particularly preferred because it provides fibers with high levels of absorbency and elasticity, is safe and non-irritating to human skin, and produces stable crosslink bonds. In addition, citric acid is available in large quantities at relatively low prices, making it commercially suitable for use as a cross-linking agent.

Another group of preferred crosslinking agents for use in the present invention comprises saturated C₂-C₉ polycarboxylic acids containing at least one oxygen atom in the chain to which the carboxyl groups are attached. Examples of such compounds include oxydisuccinic acid, tartrate monosuccinic acid having the structural formula:

and tartrate disuccinic acid with the structural formula

A more detailed description of tartrate monosuccinic acid, tartrate disuccinic acid, and salts thereof can be found in US Pat. No. 4,663,071, Bush et al., issued May 5, 1987.

Those skilled in the art of polycarboxylic acids will appreciate that the above-described crosslinking agents based on aliphatic and alicyclic C2-C9 polycarboxylic acids may be in a variety of forms, such as the free acid form and salts thereof. Although the free acid form is preferred, all such forms are intended to be included within the scope of the invention.

The individualized, crosslinked fibers used in the absorbent structures of the present invention have an effective amount of the C2-C9 polycarboxylic acid crosslinking agent reacted with the fibers in the form of intrafiber crosslink bonds. As used herein, "effective amount of crosslinking agent" refers to an amount of crosslinking agent sufficient to effect an improvement in at least one essential absorbent property of the fibers themselves and/or the absorbent structures containing the individualized, crosslinked fibers relative to conventional, uncrosslinked fibers. An example of a essential absorbent property is the drip capacity, which is a combined measurement of the fluid absorption capacity and the fluid absorption rate of an absorbent structure. A detailed description of the method for determining the drip capacity is given later.

Unexpectedly good results are obtained particularly for absorbent pads made from individualized, crosslinked fibers reacted with from about 0.5 mole % to about 10.0 mole %, more preferably with from about 1.5 mole % to about 6.0 mole % of crosslinking agent reacted with the fibers, calculated on a molar basis of cellulose anhydroglucose.

Preferably, the crosslinking agent is contacted with the fibers in a liquid medium and under conditions such that the crosslinking agent penetrates into the interior of the individual fiber structures. However, other methods of treating with crosslinking agent, including spraying the fibers while they are in an isolated, loosened form, are also within the scope of the invention.

Applicants have discovered that the crosslinking reaction can be carried out at reasonable rates without a catalyst provided the pH is maintained within a specific range (discussed in detail below). This is in contrast to the prior art which teaches that special catalysts are required to provide sufficiently rapid esterification and crosslinking of fibrous cellulose using polycarboxylic acid crosslinking agents to achieve economic operation. See, for example, U.S. Patent 4,820,307, Welch et al., issued April 11, 1989.

However, if desired, the fibers may also be contacted with a suitable catalyst prior to crosslinking. Applicants have found that the type and amount of catalyst, as well as the method of contacting it with the fibers, will depend on the particular crosslinking process being performed. These variables are discussed in greater detail later.

Once the fibers are treated with the crosslinking agent and with the catalyst, if one is used, the reaction of the crosslinking agent with the fibers is essentially in the absence of interfiber bonds, ie that interfiber contact is maintained to a low degree relative to un-fluffed pulp fibers, or the fibers are immersed in a solution which does not facilitate the formation of interfiber bonds, particularly hydrogen bonds. This results in the formation of crosslink bonds which are intrafiber bonds in nature. Under these conditions, the crosslinking agent reacts to form crosslink bonds between hydroxyl groups of a single cellulose chain or between hydroxyl groups of adjacently located cellulose chains of a single cellulose fiber.

Although not stated for the purpose of limiting the scope of the invention, nor is such a limitation intended, it is believed that the carboxyl groups on the polycarboxylic acid crosslinking agent react with the hydroxyl groups of the cellulose to form ester bonds. The formation of ester bonds, which are believed to be the desirable type of bonding to produce stable crosslink bonds, is favored under acidic reaction conditions. Therefore, acidic crosslinking conditions, i.e., pH ranges of from about 1.5 to about 5, are highly preferred for the purposes of this invention.

Prior to reacting the crosslinking agent with the fibers, the fibers are preferably mechanically defibrated into a low density, individualized, fibrous form known as "fluff." Mechanical defibration can be accomplished by a variety of methods now known in the art or which will become known hereinafter. Mechanical defibration is preferably accomplished by a method in which knotting and fiber damage are minimized. One type of device which has been found to be particularly suitable for defibrating the cellulosic fibers is the three-stage fluffing device described in U.S. Patent 3,987,968, issued to DR Moore and OA Shields on October 26, 1976, which patent is specifically incorporated herein by reference. The fluffing device described in U.S. Patent 3,987,968 subjects the wet cellulosic pulp fibers to a combination of mechanical shock, mechanical agitation, air agitation and a limited amount of air drying to form a substantially knot-free fluff. The individualized fibers are here imparted an increased amount of crimp and twist relative to the amount of crimp and twist naturally present in such fibers. This additional crimp and twist is believed to increase the elastic character of absorbent structures made from the finished crosslinked fibers.

Other methods useful for defibrating the cellulosic fibers include, but are not limited to, treatment with a Waring blender and tangential contact of the fibers with a rotating disk refiner or wire brush. Preferably, during such defibration, a stream of air is directed against the fibers to assist in separating the fibers into substantially individual shapes.

Regardless of the particular mechanical device used to form the fluff, the fibers are preferably mechanically treated to initially contain at least about 20% moisture, and preferably between about 40% and about 65% moisture.

Mechanical refining of high consistency or partially dried fibers may also be used to impart crimp or twist to the fibers in addition to the crimp or twist formed as a result of mechanical defibration.

The fibers made according to the present invention have unique combinations of stiffness and resilience which enable the absorbent structures made from the fibers to maintain high levels of absorbency and exhibit high levels of resilience and expansion responsiveness upon wetting a dry, compressed absorbent structure. In addition to having levels of crosslinking within the specified ranges, the crosslinked fibers are characterized by having water retention values (WRVs) of less than about 60, preferably from about 28 to about 50, and more preferably between about 30 and about 45 for conventional chemically pulped papermaking fibers. The WRV of a particular fiber is a measure of the degree of crosslinking. Very highly crosslinked fibers, such as those made by many of the known prior art crosslinking processes discussed above, have exhibited WRV values of less than about 25 and generally less than about 20. The particular crosslinking process used will, of course, affect the WRV of the crosslinked fiber. However, it is believed and intended that any process which results in crosslinking levels and WRV values within the specified limits is within the scope of this invention. Useful methods of crosslinking include dry crosslinking processes and non-aqueous solution crosslinking processes, as generally discussed in the Background of the Invention section. Certain preferred dry crosslinking processes and non-aqueous solution crosslinking processes for producing the individualized, crosslinked fibers of the present invention are discussed in more detail later. Aqueous solution crosslinking processes in which the solution is a cause of the fibers becoming excessively swollen will result in fibers having WRV values in excess of 60. These fibers will provide insufficient stiffness and elasticity for the purposes of the present invention.

With particular reference to dry crosslinking processes, individualized crosslinked fibers can be prepared by such a process by providing a quantity of cellulosic fibers, contacting a slurry of the fibers with a crosslinking agent of the type and amount described above, mechanically separating the fibers into substantially individual form, e.g., defibrating, and drying the fibers, and causing the crosslinking agent to react with the fibers in the presence of a catalyst to form crosslink bonds while maintaining the fibers in substantially individual form. The defibrating step is believed to impart additional crimp, apart from the drying step. Subsequent drying is accompanied by twisting of the fibers, the extent of twisting being determined by the crimped geometry of the fiber. is reinforced. As used herein, the term fiber "crimp" refers to a geometric curvature of the fiber about the long axis of the fiber. "Twist" refers to a rotation of the fiber about the perpendicular cross-section of the long axis of the fiber. The fibers of the preferred embodiment of the present invention are individualized, cross-linked in intrafiber bond form, and highly twisted and crimped.

As used herein, the term "twist number" refers to the number of twist nodes present in a given length of fiber. Twist number is used as a means of measuring the extent to which a fiber is twisted about its long axis. The term "twist node" refers to a substantially 180º axial rotation about the long axis of the fiber, whereby a portion of the fiber (i.e., the "node") appears dark relative to the rest of the fiber when viewed through a transmitted light microscope. The distance between nodes corresponds to an axial rotation of 180º. Those skilled in the art will appreciate that the appearance of a twist node as described above is primarily a visual rather than a physical phenomenon. However, the number of twist knots on a given length of fiber (i.e., the twist number) is a direct measure of the extent of fiber twist, which in turn is a physical parameter of the fiber. The occurrence and amount of twist knots will depend on whether the fiber is a summerwood fiber or a springwood fiber. The twist knots and the total twist number are determined by a twist number image analysis method described in the Experimental Methods section of the disclosure. The average twist number referred to in describing the fibers of the invention is uniquely determined by the twist number method mentioned above. In counting the twist knots, those parts of the fiber that are darkened by fiber damage or fiber compression should be distinguished from those fiber parts that appear dark due to fiber twist.

The actual twist number of any given fiber sample is determined by the ratio of spring wood fibers to summerwood fibers. The twist number of any particular springwood or summerwood fibers will also vary from fiber to fiber. Nevertheless, the average twist number limitations are convenient for defining the invention, and these limitations apply regardless of the particular combination of springwood fibers with summerwood fibers. That is, any set of fibers having twist numbers that are within the stated twist number limitations are intended to be within the scope of the present invention, provided the other claimed limitations are met.

When measuring the twist number for a fiber sample, it is important that a sufficient amount of fiber be tested to accurately represent the average of the various individual fiber twist levels. It is suggested that at least five (5) inches of cumulative fiber length from a representative sample of a mass of fibers be tested to obtain a representative value for the fiber twist number.

The twist number of the wet fiber is described and measured analogously to the twist number of the dry fiber, whereby this procedure is only modified in that the fiber is wetted with water before the examination and the twist nodes are then counted in the wet state of the fiber in accordance with the twist number image analysis procedure.

Preferably, the average twist number of the dry fiber is at least about 2.5 twist knots per millimeter and the average twist number of the wet fiber is at least about 1.5 twist knots per millimeter and is at least 1.5 twist knots per millimeter less than the twist number of the dry fiber. Most preferably, the average twist number of the dry fiber is at least about 3.0 twist knots per millimeter and the average twist number of the wet fiber is at least about 2.0 twist knots per millimeter and is at least 1.0 twist knot per millimeter less than the twist number of the dry fiber.

In addition to the fact that the fibers according to the present invention are twisted, they are also crimped. Fiber curl can be described as a proportional shortening of the fiber due to kinks, twists and/or bends in the fiber. For the purposes of this disclosure, fiber curl shall be measured from the standpoint of a two-dimensional field. The amount of fiber curl is referred to by a fiber curl index. The fiber curl factor, a two-dimensional measurement of curl, is determined by viewing the fiber in a two-dimensional plane, measuring the projected length of the fiber as the longest dimension of a rectangle circumscribing the fiber, LR, and the actual length of the fiber, LA, and then calculating the fiber curl factor from the following equation:

(1) Curl factor = (LA/LR) - 1

A fiber curl index image analysis method is used to measure LR and LA. This method is described in the Experimental Methods section of this disclosure. The background information for this method is described in the 1979 International Paper Physics Conference Symposium, The Harrison Hotel, Harrison Hot Springs, British Columbia, September 17-19, 1979, in a paper entitled "Application Of Image Analysis To Pulp Fibre Characterization: Part 1," by B. D. Jordan and D. H. Page, pp. 104-114, Canadian Pulp and Paper Association (Montreal, Quebec, Canada), which reference is specifically incorporated herein by reference.

Preferably, the fibers have a crimp factor of at least about 0.30, and more preferably at least about 0.50.

When the fibers are maintained in substantially individual form during drying and crosslinking, the fibers are allowed to twist during drying and thereby become crosslinked in such a twisted, curled state. Drying the fibers under conditions such that the fibers can twist and curl is referred to as drying the fibers under substantially unconstrained conditions. On the other hand, drying the fibers in sheet form results in dried fibers that are not as twisted and are curled like fibers that have been dried in substantially individualized form. Interfiber hydrogen bonding is believed to "limit" the relative occurrence of twist and curl of the fiber.

There are several methods by which the fibers can be contacted with the crosslinking agent and catalyst (if a catalyst is used). In one embodiment, the fibers are contacted with a solution initially containing both the crosslinking agent and catalyst. In another embodiment, the fibers are contacted with an aqueous solution of the crosslinking agent and immersed prior to the addition of the catalyst. The catalyst is then added. In a third embodiment, the crosslinking agent and catalyst are added to an aqueous slurry of the cellulosic fibers. Other methods in addition to those described herein will be apparent to those skilled in the art and are intended to be encompassed within the scope of this invention. Regardless of the particular method by which the fibers are contacted with the crosslinking agent and catalyst (if a catalyst is used), the cellulosic fibers, crosslinking agent and catalyst are preferably mixed and/or sufficiently agitated to allow the fibers to make thorough contact and to impregnate the individual fibers.

Applicants have discovered that the crosslinking reaction can be achieved without the use of a catalyst if the pH of the solution containing the crosslinking agent is maintained within the ranges specified below. In particular, the aqueous portion of the cellulose fiber slurry or crosslinking agent solution should be adjusted to a target pH of between about pH 1.5 and about pH 5, more preferably between about pH 2.0 and about pH 3.5, during the contact period between the crosslinking agent and the fibers. Preferably, the pH is adjusted by adding a base, such as sodium hydroxide, to the crosslinking agent solution.

Regardless of the above, in general any substance that can catalyze the cross-linking mechanism can be used Useful catalysts include alkali metal hypophosphites, alkali metal phosphites, alkali metal polyphosphates, alkali metal phosphates, and alkali metal sulfates. Particularly preferred catalysts are the alkali metal hypophosphites, alkali metal phosphates, and alkali metal sulfates. The mechanism of catalysis is unknown, although applicants believe that the catalysts may simply act as buffering agents, maintaining pH values within desired ranges. A more complete list of catalysts useful herein can be found in U.S. Patent 4,820,307, Welch et al., issued April 11, 1989, which is incorporated herein by reference. The selected catalyst may be used as the sole catalytic agent or it may be used in combination with one or more other catalysts.

The amount of catalyst preferably used will, of course, depend on the particular type and amount of crosslinking agent and the reaction conditions, particularly temperature and pH. In general, based on technical and economic considerations, catalyst amounts of between about 5% by weight and about 80% by weight, based on the weight of crosslinking agent added to the cellulosic fibers, are preferred. For exemplary purposes, in the case where the catalyst used is sodium hypophosphite and the crosslinking agent is citric acid, a catalyst amount of about 50% by weight, based on the amount of citric acid added, is preferred. In addition, it is desirable to adjust the aqueous portion of the cellulosic fiber slurry or crosslinking agent solution to a target pH between about pH 1.5 and about pH 5, more preferably between about pH 2.0 and about pH 3.5, during the period of contact between the crosslinking agent and the fibers.

The cellulosic fibers should generally be dewatered and optionally dried. The usable and optimal consistencies will depend on the type of fluffing device used. In the preferred embodiments, the cellulosic fibers are dewatered and optimally dried to a consistency between about 20% and about 80%. More preferably, the fibers are dewatered and dried to a consistency level of between about 35% and about 60%. Drying the fibers to these preferred ranges will generally facilitate defibration of the fibers into individualized form without excessive knotting associated with higher moisture contents and without higher levels of fiber damage associated with lower moisture contents.

For exemplary purposes, dewatering can be accomplished by such methods as mechanically pressing, centrifuging, or air drying the pulp. Additional drying of the fibers within the 35 to 60% consistency range described above can optionally be accomplished, but is preferably accomplished by a process known in the art as air drying and is performed under conditions where the use of high temperature for an extended period of time is not required. Excessively high temperature and time in this step can result in drying of the fibers beyond 60% consistency, potentially causing excessive fiber damage during the subsequent defibration step. After dewatering, the fibers are then mechanically defibrated as previously stated.

The defibrated fibers are then dried to a consistency between 60% and 100% in a process known in the art as flash drying. This step imparts additional twist and curl to the fibers while removing the water from them. Although the amount of water removed by this additional drying step may vary, it is believed that flash drying to higher consistency results in a greater degree of fiber twist and curl than flash drying to a consistency in the lower part of the 60%-100% range. In the preferred embodiments, the fibers are dried to a consistency of about 90%-95%. It is believed that this degree of flash drying results in the desired degree of fiber twist and curl without requiring higher flash drying temperatures and residence times required to achieve 100% consistency. Flash drying the fibers to a consistency of about 90%-95% in the higher part of the 60%-100% range also reduces the amount of drying that must be achieved in the curing step following flash drying.

The flash dried fibers are then heated to an appropriate temperature for an effective period of time to cause the crosslinking agent to cure, i.e., react with the cellulosic fibers. The rate and extent of crosslinking will depend on the dryness of the fibers, the temperature, the pH, the amount and type of catalyst and crosslinking agent, and the methods used to heat and/or dry the fibers while crosslinking is carried out. Crosslinking at a particular temperature will occur at a faster rate for fibers of a particular initial moisture level if continuous through-air drying is concurrent than if the fibers are subjected to drying/heating in a static oven. Those skilled in the art will recognize that a number of temperature-time relationships exist for curing the crosslinking agent. Drying temperatures of from about 145°C to about 165°C for periods of between about 30 minutes and 60 minutes under static atmospheric conditions will generally provide acceptable curing effects for fibers having moisture contents of less than about 10%. Those skilled in the art will also recognize that higher temperatures and forced air convection will reduce the time required for curing. Therefore, drying temperatures of from about 170°C to about 190°C for periods of between about 2 minutes and 20 minutes in a through-flow oven will also generally provide acceptable curing effects for fibers having moisture contents of less than about 10%. Curing temperatures should be maintained at less than 225ºC, preferably less than about 200ºC, since exposure to such high temperatures may cause darkening or other damage to the fibers.

Without being bound by theory, it is believed that the chemical reaction of the cellulosic fibers with the C₂-C₉ polycarboxylic acid crosslinking agent will not begin until the mixture of these materials is heated in the curing oven. During the curing step, the ester crosslink bonds are formed between the C2 -C9 polycarboxylic acid crosslinking agent and the cellulose molecules. These ester crosslinks are mobile under the influence of heat because of a transesterification reaction that takes place between ester groups and adjacent unesterified hydroxyl groups on the cellulose fibers. It is further believed that the process of transesterification that takes place after the initial ester bonds are formed results in fibers that have improved absorbent properties compared to fibers that are not sufficiently cured to allow transesterification to occur.

Following the crosslinking step, the fibers are washed, if desired. After washing, the fluid is removed from the fibers and the fibers are dried. The fibers, while still in a wet state, may be subjected to a second mechanical defibration step between the fluid removal and drying steps, which causes twisting and curling of the crosslinked fibers. The same apparatus and methods previously described for defibration of the fibers are applicable in this second mechanical defibration step. As used in this paragraph, the term "defibration" refers to any method that can be used to mechanically separate the fibers into substantially individual forms, even though the fibers may already be provided in such forms. "Defibration" therefore refers to the step of mechanically treating the fibres, either in individual form or in a more compacted form, such mechanical treatment step a) separating the fibres into substantially individual form if they are not already in such form, and b) causing the fibres to curl and twist upon drying.

This second defibration treatment after the fibers have been cross-linked is believed to enhance the twisted, curled character of the pulp. This enhancement of the twisted, curled configuration of the fibers results in to increased elasticity of the absorbent structure and to a responsiveness when moistened.

The maximum extent of crosslinking is achieved when the fibers are substantially dry (containing less than about 5% moisture). This lack of water causes the fibers to be crosslinked while in a substantially unswollen, collapsed state. As a result, they typically have low fluid retention values (FRV) relative to the range useful in this invention. The FRV refers to the amount of fluid, calculated on a dry fiber basis, that remains absorbed by a fiber sample after it has been immersed and then centrifuged to separate the interfiber fluid. (The further definition of FRV and the method for determining FRV are described later.) The amount of fluid that the crosslinked fibers can absorb depends on their ability to swell at saturation, or in other words, their internal diameter or volume when swollen to a maximum level. This, in turn, depends on the extent of crosslinking. As the degree of intrafiber crosslinking increases for a given fiber and process, the FRV of the fiber will decrease. Thus, the FRV value of a fiber is a structural description of the physical state of the fiber at saturation. Unless specifically stated otherwise, the FRV values described herein are intended to be reported as water retention values (WRV) of the fibers. Other fluids, such as salt water and synthetic urine, may also be advantageously used as the fluid medium for analysis. In general, the FRV of a particular fiber crosslinked by processes in which cure is largely dependent on drying, such as the present process, will depend primarily on the crosslinking agent and the extent of crosslinking. The WRV values of the fibers crosslinked by this dry crosslinking process with amounts of crosslinking agent applicable to this invention are generally less than about 60, greater than about 28, preferably less than about 50, and more preferably between about 30 and about 45. Bleached SSK fibers containing from about 1.5 mole percent to about 6.0 mole percent citric acid calculated on a molar Based on cellulose anhydroglucose contained in reacted form with the fibers were observed to have WRV values ranging from about 28 to about 40, respectively. The extent of bleaching and the performance of bleaching steps after crosslinking were found to be criteria affecting WRV. Southern Softwood Kraft (SSK) fibers produced in many known prior art crosslinking processes have crosslinking levels higher than those described herein and have WRV values of less than about 25. Such fibers were observed to be extremely stiff, as discussed above, and to exhibit lower absorbency capabilities than the fibers of the present invention.

In another method of making individualized, crosslinked fibers by dry crosslinking, cellulosic fibers are contacted with a solution containing a crosslinking agent as described above. Either before or after the fibers are contacted with the crosslinking agent, they are formed into a sheet-like form. While in sheet form, the fibers are dried and crosslinked, preferably by heating the fibers to a temperature between about 120°C and about 160°C. Following crosslinking, the fibers are mechanically separated into substantially individualized form. This is preferably accomplished by treatment with a fiber fluffing device such as that described in U.S. Patent 3,987,968, or may be accomplished by other methods for defibrating fibers as are known in the art. The individualized crosslinked fibers produced by this sheet crosslinking process are treated with a sufficient amount of crosslinking agent such that an effective amount of the crosslinking agent, preferably between about 0.5 mole percent and about 10.0 mole percent crosslinking agent, calculated on a molar basis of cellulose anhydroglucose and measured following defibration, reacts with the fibers in the form of intrafiber crosslink bonds. Another effect of drying and crosslinking the fibers while in sheet form is that fiber-to-fiber bonding prevents the fibers from separating as they are dried. to twist and curl. As compared to individualized, crosslinked fibers made by a process in which the fibers are dried under substantially unconfined conditions and then crosslinked in a twisted, curled configuration, it is expected that absorbent structures containing the relatively untwisted fibers made by the sheet curing process described above will exhibit lower wet resiliency and lower responsiveness to wetting.

It is also contemplated that the fibers may be mechanically separated into substantially individual form between the drying step and the crosslinking step. That is, the fibers are contacted with the crosslinking agent and then dried while in sheet form. Prior to crosslinking, the fibers are individualized to facilitate intrafiber crosslinking. This other crosslinking method, as well as other variations apparent to those skilled in the art, are intended to be within the scope of this invention.

Another category of crosslinking processes applicable to the present invention are non-aqueous solution cure crosslinking processes. The same types of fibers that are useful for dry crosslinking processes can be used to make fibers that are crosslinked in non-aqueous solution. The fibers are treated with a sufficient amount of crosslinking agent so that an effective amount of crosslinking agent subsequently reacts with the fibers and are optionally treated with a suitable catalyst. The amounts of crosslinking agent and catalyst (if used) used will depend on the reaction conditions such as consistency, temperature, water content in the crosslinking solution and in the fibers, type of crosslinking agent and diluent in the crosslinking solution and the degree of crosslinking desired. The crosslinking agent is reacted while the fibers are immersed in a substantially non-aqueous solution. The non-aqueous crosslinking solution contains a non-aqueous, water-miscible polar diluent such as, but not limited to, acetic acid, propanoic acid, or acetone. The crosslinking solution may also contain a limited amount of water or other fiber-swelling liquid, but preferably the amount of water is not sufficient to cause a significant amount of fiber swelling. Crosslinking solution systems suitable for use as a crosslinking medium include those disclosed in U.S. Patent No. 4,035,147, issued to S. Sangenis, G. Guiroy, and J. Quere on July 12, 1977, which patent is hereby specifically incorporated by reference.

The crosslinked fibers used in the absorbent structures of the present invention are preferably prepared by the dry crosslinking process discussed above. The crosslinked fibers can be used directly to make airlaid absorbent cores. In addition, because of their stiffened and resilient character, the crosslinked fibers can be wetlaid into an uncompacted, low density sheet which, when subsequently dried, is directly usable as an absorbent core without further mechanical treatment. The crosslinked fibers can also be wetlaid into compacted pulp sheets for sale or transportation to remote locations.

Relative to pulp sheets made from conventional, uncrosslinked cellulosic fibers, pulp sheets made from the crosslinked fibers of the present invention are more difficult to compress to conventional pulp sheet densities. Therefore, it may be desirable to combine crosslinked fibers with uncrosslinked fibers such as those commonly used to make absorbent cores. Pulp sheets containing stiffened, crosslinked fibers preferably contain between about 5% and about 90% uncrosslinked cellulosic fibers, based on the total dry weight of the sheet, in admixture with the individualized, crosslinked fibers. It is particularly preferred to include from about 5% to about 30% highly refined, uncrosslinked cellulosic fibers, based on the total dry weight of the sheet. Such highly refined Fibers are refined or beaten to a degree of freedom of less than about 300 ml CSF (Canadian Standard Freedom), preferably to less than 100 ml CSF. The uncrosslinked fibers are preferably mixed with an aqueous slurry of the individualized, crosslinked fibers. This mixture can then be formed into a densified pulp sheet for subsequent defibration and formation into absorbent pads. The incorporation of uncrosslinked fibers facilitates compression of the pulp sheet into a densified form while resulting in surprisingly little loss of absorbency for the subsequently formed absorbent pads. In addition, the uncrosslinked fibers increase the tear strength of the pulp sheet and absorbent pads made either from the pulp sheet or directly from the mixture of crosslinked and uncrosslinked fibers. Regardless of whether the mixture of crosslinked and uncrosslinked fibers is first formed into a pulp sheet and then formed into an absorbent pad or whether it is directly formed into an absorbent pad, the absorbent pad can be laid in the air stream or wet laid.

Sheets or webs made from the individualized, crosslinked fibers or from blends also containing uncrosslinked fibers will preferably have basis weights of less than about 800 gsm and densities of less than about 0.60 gsm. Although not intended to limit the scope of the invention, wetlaid sheets having basis weights between 300 gsm and about 600 gsm and having densities between 0.07 gsm and about 0.30 gsm are particularly contemplated for direct application as absorbent cores in disposable articles such as diapers, tampons and other catamenial products. Structures with basis weights and densities higher than these specifications are believed to be particularly suitable for subsequent shredding and air-laying or wet-laying to form a lower density and basis weight structure more suitable for absorbent product applications. Nevertheless, such structures with higher basis weight and higher density, they also exhibit surprisingly high absorbency and responsiveness to wetting. Other applications contemplated for the fibers of the present invention include low density tissue sheets having density values that may be less than about 0.03 g/cm³.

If desired, the crosslinked fibers may be further processed to remove excess unreacted crosslinking agent. A number of treatments that have been found to be successful in removing excess crosslinking agent include subsequently washing the crosslinked fibers, allowing the fibers to soak in an aqueous solution for a substantial period of time, sieving the fibers, dewatering the fibers, e.g., by centrifugation, to a consistency of between about 40% and about 80%, mechanically defibrating the dewatered fibers as described above, and air drying the fibers. If necessary, a sufficient amount of an acidic substance may be added to the wash solution to maintain the wash solution at a pH of less than about 7. Without being bound by theory, it is believed that the ester crosslinks are not stable under alkaline conditions and that maintaining the pH in the acidic range during the washing treatment prevents reversal of the ester crosslinks formed. Acidic conditions can be created by mineral acids such as sulphuric acid or alternatively in the form of acidic bleaching chemicals such as chlorine dioxide and sodium hydrosulphite (which can also be added to brighten the crosslinked fibres). This process has been found to reduce the free crosslinking agent content to between about 0.01% and about 0.15%.

The crosslinked fibers described herein are useful in a variety of absorbent articles including, but not limited to, tissue sheets, disposable diapers, catamenials, sanitary napkins, tampons, and bandages, in which each of the articles includes an absorbent structure containing the individualized crosslinked fibers described herein. For example, a disposable A diaper or similar article having a liquid permeable topsheet, a liquid impermeable backsheet in association with the topsheet, and an absorbent structure containing individualized, crosslinked fibers is particularly contemplated. Such articles are generally described in U.S. Patent No. 3,860,003, issued to Kenneth B. Buell on January 14, 1975, which is specifically incorporated herein by reference.

Typically, absorbent cores for diapers and menopausal hygiene articles are made from unstiffened, non-crosslinked cellulose-based fibers, with the absorbent cores having dry densities of about 0.06 g/cm³ and about 0.12 g/cm³. When moistened, the absorbent core generally undergoes a reduction in volume.

It has been found that the crosslinked fibers of the present invention can be used to make absorbent cores that have significantly higher fluid absorption properties, including but not limited to absorbent capacity and wicking rate, relative to absorbent cores of equivalent density made from conventional, uncrosslinked fibers or crosslinked fibers of the prior art. In addition, these improved absorption results can be achieved in conjunction with increased levels of wet resiliency. For absorbent cores having densities of between about 0.05 g/cc and about 0.15 g/cc that maintain a substantially constant volume when wetted, it is particularly preferred to use crosslinked fibers having crosslinking levels of between about 5.0 mole percent and about 10.0 mole percent crosslinking agent on a dry cellulose anhydroglucose molar basis. Absorbent cores made from such fibers exhibit a desirable combination of structural integrity, ie, resistance to compression, and wet resilience. The term wet resilience in the present context refers to the ability of a moistened pad to return to its original shape and volume upon application and release of compression forces. Compared to cores made from untreated fibers and cross-linked fibers of the state of the art, technology, the absorbent cores made from fibers according to the invention will regain a substantially higher proportion of their original volume when the wet compression forces are released.

In another preferred embodiment, the individualized, crosslinked fibers are formed into either an air-laid or wet-laid (and subsequently dried) absorbent core that is compressed to a dry density less than the equilibrium wet density of the pad. The equilibrium wet density is the density of the pad calculated based on the dry fiber at which the pad is fully saturated with fluid. If the fibers are formed into an absorbent core with a dry density less than the equilibrium wet density, the core will collapse to the equilibrium wet density upon wetting to saturation. On the other hand, if the fibers are formed into an absorbent core with a dry density greater than the equilibrium wet density, the core will expand to the equilibrium wet density upon wetting to saturation. Pads made from the fibers of the invention have equilibrium wet densities that are significantly lower than those of pads made from conventional fluffy fibers. The fibers of the invention can be compressed to a density higher than the equilibrium wet density to form a thin pad that will expand when moistened, thereby increasing the absorbent capacity to a level significantly greater than can be achieved for uncrosslinked fibers.

In another preferred embodiment, high absorbent properties, wet resilience and responsiveness to wetting can be achieved at crosslinking levels between about 1.5 mole percent and about 6.0 mole percent, calculated on a molar basis of dry cellulose. Preferably, such fibers are formed into absorbent cores having dry densities greater than equilibrium wet densities. Preferably, the absorbent cores are compressed to densities between about 0.12 g/cm³ and about 0.60 g/cm³, with the corresponding Equilibrium wet density is less than the density of the dry compressed pad. Also preferably, the absorbent cores are compressed to a density between about 0.12 g/cc and about 0.40 g/cc, with corresponding equilibrium wet densities between about 0.08 g/cc and about 0.12 g/cc and lower than the densities of the dry compressed cores. It will be appreciated, however, that absorbent structures within the higher density range can be made from crosslinked fibers having higher levels of crosslinking, just as lower density absorbent structures can be made from crosslinked fibers having lower levels of crosslinking. Improved performance is achieved for all such structures relative to individualized crosslinked fibers of the prior art.

While the foregoing discussion deals with preferred embodiments for high and low density absorbent structures, it should be noted that a variety of combinations of density values and crosslinking agent levels of absorbent structures within the ranges disclosed herein will provide superior absorbent properties and improved integrity of the absorbent structure relative to conventional cellulosic fibers and known crosslinked fibers of the prior art. Such embodiments are intended to be included within the scope of this invention.

Absorbent structures made from individualized, cross-linked fibers may additionally contain discrete particles of substantially water-insoluble hydrogel-forming material. Hydrogel-forming materials are chemical compounds capable of absorbing fluids and retaining them under moderate pressures.

Suitable hydrogel-forming materials may be inorganic materials such as silica gels or organic compounds such as cross-linked polymers. It is to be understood that the term cross-linking, when used in connection with hydrogel-forming materials, takes on a broader meaning than when used in connection with the reaction of cross-linking agents with cellulosic fibers to form individualized, crosslinked fibers. Crosslinked hydrogel-forming polymers can be crosslinked by covalent, ionic, van der Waals, or hydrogen bonding. Examples of hydrogel-forming materials include polyacrylamides, polyvinyl alcohol, ethylene-maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, carboxylmethyl cellulose, polyvinylmorpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinylpyridine, and the like. Other suitable hydrogel-forming materials are those disclosed in U.S. Patent 3,901,236 to Assarsson et al., issued August 26, 1975, which disclosure is incorporated herein by reference. Particularly preferred hydrogel-forming polymers for use in the absorbent cores are hydrolyzed acrylonitrile-grafted starch, acrylic acid-grafted starch, polyacrylates and isobutylene-maleic anhydride copolymers, or mixtures thereof. Examples of hydrogel-forming materials that may be used are Aqualic L-73, a partially neutralized polyacrylic acid manufactured by Nippon Shokubai Co., Japan, and Sanwet IM 1000, a partially neutralized acrylic acid-grafted starch manufactured by Sanyo Co., Ltd., Japan. Hydrogel-forming materials having relatively high gel strengths, such as those described in U.S. Patent Application Ser. No. 746,152, filed June 18, 1985, which is specifically incorporated herein by reference, are preferred for use with individualized, crosslinked fibers.

Methods for preparing hydrogel-forming materials are disclosed in Masuda et al., U.S. Patent 4,076,663, issued February 28, 1978; in Tsubakimoto et al., U.S. Patent 4,286,082, issued August 25, 1981; and further in U.S. Patents 3,734,876, 3,661,815, 3,670,731, 3,664,343, 3,783,871, and Belgian Patent 785,850, all of which disclosures are incorporated herein by reference.

The hydrogel-forming material may be distributed throughout an absorbent structure containing individualized, cross-linked fibers, or it may be limited to distribution within a specific layer or portion of the absorbent structure. In another embodiment the hydrogel-forming material is adhered to or laminated to a sheet or film applied to a fibrous absorbent structure which may contain individualized, cross-linked fibers. Such a sheet or film may be multilayered so that the hydrogel-forming material is contained between the layers. In another embodiment, the hydrogel-forming material may be adhered directly to the surface fibers of the absorbent structure.

Surprisingly large increases in skin dryness have been observed in absorbent structures combining the individualized, cross-linked fibers of the present invention and hydrogel-forming materials, as determined by the level of skin wetness measured with an evaporimeter following contact of the wet absorbent structures with human skin. This improvement is believed to be due to the high wicking of the individualized, cross-linked fibers relative to conventional fibers and the increased absorbent capacity of the structure. The unique absorbency of structures made from individualized, cross-linked fibers is due to the rigid nature of the fibers and the relatively large void spaces that result. However, excessive levels of cross-linking agent, such as may be present in certain prior art individualized, cross-linked fibers, can reduce wicking due to the hydrophobic properties of the cross-linking agent.

Another significant advantage has been observed with respect to absorbent structures made from individualized, cross-linked fibers with dry densities higher than their corresponding equilibrium wet densities (calculated on a dry fiber basis). In particular, this type of absorbent structure expands in volume upon wetting. As a result of this expansion, the capillary interfiber network of the fibers will also expand. In conventional absorbent structures containing hydrogel-forming material mixed in, the hydrogel-forming material expands in volume upon fluid absorption and can close the capillary pathways. for fluid absorption before the full fluid absorption potential of the structure is utilized. This phenomenon is known as gel blocking. Capillary enlargement through expansion of the fibrous network of the absorbent structure reduces the occurrence of gel blocking. This allows greater portions of the structure's fluid absorption potential to be utilized and allows greater amounts of hydrogel-forming material (if desired) to be incorporated into the absorbent structure without significant levels of gel blocking occurring.

Absorbent structures containing individualized, cross-linked fibers and hydrogel-forming material intended for diaper core applications preferably have dry densities of between about 0.15 g/cc and about 0.40 g/cc and preferably contain less than about 20% hydrogel-forming material calculated on a dry fiber weight basis. Most preferably, the individualized, cross-linked fibers comprise between about 1.5 mole percent and about 6.0 mole percent citric acid calculated on a molar basis of cellulose anhydroglucose reacted therewith in the form of intrafiber cross-link bonds, whereby the fibers are formed into an absorbent structure that is relatively thin when sufficiently compressed dry so that the structure can expand upon wetting.

The hydrogel-forming material may be homogeneously distributed throughout all or a portion of the absorbent structure. Indeed, a diaper structure as disclosed in U.S. Patent 3,860,003 having an absorbent core containing the preferred individualized, crosslinked fibers has a dry density of about 0.20 g/cc and also contains hydrogel-forming material distributed throughout the core. It is currently believed that an optimum balance of diaper absorbency, total absorbent capacity, skin moisture and economic feasibility is achieved at levels of between about 5% and about 20% by weight, based on the total weight of the dry absorbent core, of a hydrogel-forming material such as Aqualic L-73. Preferably, between about 8% and about 10% by weight of hydrogel-forming material is homogeneously mixed with the absorbent cores containing the individualized, cross-linked fibers in products as disclosed in U.S. Patent No. 3,860,003.

The absorbent structures described above may also contain conventional fluffy fibers or highly refined fibers, with the amount of hydrogel-forming material being based on the total weight of the fibers, as previously stated. The embodiments disclosed herein are exemplary in nature and are not intended to limit the scope of application of hydrogel-forming materials with individualized, cross-linked fibers.

METHOD FOR DETERMINING FLUID RETENTION VALUE

The following procedure can be used to determine the water retention value for cellulosic fibers.

A sample of about 0.3 g to about 0.4 g of the fibers is allowed to soak in a lidded container with about 100 ml of distilled or deionized water at room temperature for about 15 to about 20 hours. The soaked fibers are collected on a filter paper and transferred to an 80 mesh wire basket placed about 1 1/2 inches above a 60 mesh sieve bottom of a centrifuge tube. The tube is covered with a plastic lid and the sample is centrifuged for 19 to 21 minutes at a relative centrifuge force of 1500 to 1700 g. The centrifuged fibers are then removed from the basket and weighed. The weighed fibers are dried to a constant weight at 105ºC and reweighed. The water retention value is calculated as follows:

(1) WRV = (W-D)/D x100

wherein

W = wet weight of centrifuged fibres;

D = dry weight of fibres; and

W-D = weight of water absorbed.

METHOD FOR DETERMINING DRIP CAPACITY

The following procedure can be used to determine the dripping capacity of absorbent cores. The dripping capacity is used as a combined measure of the absorbent capacity and the absorption rate of the cores.

A four inch by four inch absorbent pad weighing approximately 7.5 g is placed on a mesh screen. Synthetic urine is applied to the center of the pad at a rate of 8 ml/sec. The flow of synthetic urine is stopped when the first drop of synthetic urine emerges from the bottom or sides of the pad. The drip capacity is calculated as the difference in the masses of the pad before and after application of synthetic urine divided by the mass of the fibers based on bone dry conditions.

METHOD FOR DETERMINING WET COMPRESSIBILITY

The following procedure can be used to determine the wet compressibility of absorbent structures. Wet compressibility is used as a measure of resistance to wet compression, wet structural integrity and wet resilience of the absorbent cores.

A four inch by four inch square pad weighing approximately 7.5 grams is prepared, its thickness measured, and the density calculated. The pad is loaded with synthetic urine to ten times its dry weight or to its saturation point, whichever is less. A compression force of 0.1 PSI is applied to the pad. After approximately 60 seconds, during which time the pad equilibrates, the thickness of the pad is measured. The compression force is then increased to 1.1 PSI, the pad is allowed to equilibrate, and the thickness is measured. The compression force is then decreased to 0.1 PSI, the pad is allowed to equilibrate, and the thickness is measured again. Densities are calculated for the pad at the original load of 0.1 PSI, at the 1.1 PSI load, and at the second load of 0.1 PSI, the latter load being referred to as the 0.1 PSIR (PSI rebound). The void volume, which is given in cm³/g, is then calculated for each respective pressure load Void volume is the reciprocal of the wet density of the pad minus the fiber volume (0.95 cc/g). Void volumes at 0.1 PSI and 1.1 PSI are valuable indicators of wet compression resistance and wet structural integrity. Higher void volumes for typical initial pad densities mean greater wet compression resistance and greater wet structural integrity. The difference between void volumes at 0.1 PSI and 0.1 PSIR is valuable for comparing the wet resiliency of absorbent pads. A smaller difference between the 0.1 PSI void volume and the 0.1 PSIR void volume means higher wet resiliency.

It was also shown that the difference in the grip height between the dry pad and the saturated pad before compression is a valuable indicator of the responsiveness to wetting.

METHOD FOR DETERMINING DRY COMPRESSIBILITY

The following procedure can be used to determine the dry compressibility of absorbent cores. The dry compressibility is used as a measure of the dry resilience of the cores.

A four-inch by four-inch square air-laid pad weighing approximately 7.5 g is prepared and compressed in a hydraulic press while dry at 5500 lbs/16 in². The pad is turned over and the compression process repeated. The thickness of the pad is measured before and after compression using an unloaded gripper. The density before and after compression is then calculated as mass/(area x thickness). Larger differences between the density before and after compression indicate lower dry resilience.

METHOD FOR DETERMINING THE AMOUNT OF C₂-C�9 POLYCARBOXYLIC ACID REACTED WITH CELLULOSE-BASED FIBERS

There are a variety of analytical methods suitable for determining the amount of polycarboxylic acid cross-linked with the cellulosic fibres. Any suitable For purposes of determining the amount of the preferred C2-C9 polycarboxylic acid (e.g., citric acid, 1,2,3-propanetricarboxylic acid, 1,2,3,4-butanetetracarboxylic acid, and oxydisuccinic acid) that is reacted to form intrafiber crosslink bonds with the cellulosic component of the individualized, crosslinked fibers in the examples of the present invention, the following procedure is used. First, a sample of the crosslinked fibers is washed with a sufficient amount of hot water to remove any unreacted crosslinking chemicals or catalysts. The fibers are then dried to an equilibrium moisture content. The carboxyl group content of the individualized, crosslinked fibers is then determined in substantial accordance with TAPPI Method T 237 OS-77. The degree of crosslinking of the C₂-C�9 polycarboxylic acid is then calculated from the carboxyl group content of the fiber using the following formula:

wherein

C = carboxyl group content of the cross-linked fibres, meq/kg

30 = carboxyl group content of the non-crosslinked pulp fibers, meq/kg

*162 g/mol = molecular weight of the cross-linked pulp fibers (i.e. one anhydroglucose unit)

The assumptions made to derive the above formula are:

1. The molecular weight of the cross-linked fibres is equivalent to that of non-cross-linked pulp, i.e. 162 g/mol (calculated on a molar basis of cellulose anhydroglucose).

2. Two of the three carboxyl groups of citric acid react with hydroxyl groups on the cellulose to form a crosslink bond, leaving one carboxyl group free for measurement by the carboxyl test.

3. Two of the three carboxyl groups of tricarballylic acid (TCBA, also known as 1,2,3-propanetricarboxylic acid) react with two hydroxyl groups on the cellulose to form a crosslink bond, leaving one carboxyl group free for measurement by the carboxyl test.

4. Three of the four carboxyl groups of 1,2,3,4-butanetetracarboxylic acid (BTCA) react with hydroxyl groups on the cellulose to form a cross-linking bond, leaving one carboxyl group free for measurement by the carboxyl test.

5. Three of the four carboxyl groups of oxydisuccinic acid (ODS) react with hydroxyl groups on the cellulose to form a cross-linking bond, leaving one carboxyl group free for measurement by the carboxyl test.

6. Uncrosslinked pulp fibers have a carboxyl content of 30 meq/kg.

7. No new carboxyl groups are formed on the cellulose during the cross-linking process.

METHOD FOR DETERMINING THE SPEED OF TURN

The following method may be used to determine the twist number of fibers analyzed in this disclosure.

Dry fibers are placed on a slide coated with a thin film of immersion oil and then covered with a coverslip. The effect of the immersion oil is to make the fiber transparent without causing swelling and thereby aid in the identification of the twist nodes (described below). Wet fibers are placed on a slide by placing a low-consistency slurry of the fibers on the slide and then covering the slide with a coverslip. The water makes the fibers transparent, thus facilitating the identification of the twist nodes.

An image analyzer comprising a computer-controlled microscope, a video camera, a video screen, and a computer with QUIPS software, available from Cambridge Instruments Limited (Cambridge, England; Buffalo, New York), is used to determine the rotation number.

The total length of the fibers within a specific area of the microscope slide is measured by the image analyzer at 200x magnification. The twist nodes are identified and marked by an operator. The process continues with the fiber length continuing to be measured and the twist nodes continuing to be marked until 1270 mm inches of the total fiber length are analyzed. The number of twist nodes per millimeter is calculated from this data by dividing the total number of twist nodes marked by the total fiber length.

METHOD FOR DETERMINING THE CURLS FACTOR

The following method can be used to measure the curl index of the fiber.

Dry fibers are placed on a microscope slide. A cover sheet is placed on the fibers and glued to the edges. The actual length LA and the maximum projected length LR (equivalent to the length of the longest side of a rectangle circumscribing the fiber) are measured using an image analyzer comprising a software-controlled microscope, a video camera, a video monitor and a computer. The software used is the same as that described in the twist number image analysis method above.

Once LA and LR are obtained, the curl factor is calculated using equation (1) shown previously. The curl factor for each fiber sample is calculated for at least 250 individual fibers and then averaged to determine the mean curl factor for the sample. Fibers with a length of LA less than 0.25 mm are excluded from the calculation.

The following examples illustrate the practice of the present invention, but are not intended to limit the invention.

EXAMPLE I

Individualized crosslinked fibers used in the absorbent structures of the present invention are prepared in a dry crosslinking process using citric acid as the crosslinking agent. The process used to prepare the citric acid crosslinked fibers is as follows:

1. For each sample, 1.735 g of once-dried Southern Softwood Kraft (SSK) pulp is provided. The fibers have a moisture content of approximately 7% (equivalent to a consistency of 93%).

2. A slurry is formed by adding fibers to an aqueous solution containing about 2.942 g of citric acid and 410 ml of 50% sodium hydroxide solution in 59.323 g of water. The fibers are allowed to soak in the slurry for about 60 minutes. This step is also called "washing." The pH of the wash is about 3.0.

3. The fibers are then dewatered by centrifugation to a consistency in the range of about 40% to about 50%. The consistency of the centrifuged slurry in this step, combined with the concentration of carboxylic acid in the slurry filtrate in step 2, determine the amount of crosslinking agent present on the fibers after centrifugation. In this example, after the initial centrifugation, there is about 6% by weight of citric acid on the fibers, based on cellulose anhydroglucose of the dry fiber. In practice, the concentration of crosslinking agent in the slurry filtrate is calculated by assuming a desired dewatering consistency and a desired amount of chemicals on the fibers.

4. The dewatered fibers are then refined using a Sprout-Waldron 12'' disk refiner (model number 105-A) with the plates set with a gap, which essentially produces individualized fibers with minimal fiber damage, defibration. As the individualized fibers leave the refiner, they are dried by flash drying with hot air in two vertical tubes to impart fiber twist and curl. The fibers contain about 10% moisture upon exiting these tubes and are ready to be cured. If the moisture content of the fibers upon exiting the flash drying tubes is greater than about 10%, then the fibers are dried with room temperature air until their moisture content is about 10%.

5. The nearly dry fibers are then placed on panels and cured in a flow-through drying oven for a period of time and at a temperature which in practice will depend on the amount of citric acid added, the dryness of the fibers, etc. In this example, the samples are cured at a temperature of about 188ºC for a period of about 8 minutes. Cross-linking is completed during the period in the oven.

6. The cross-linked, individualized fibers are placed on a mesh screen and rinsed with water at approximately 20ºC, soaked in water at approximately 60ºC for one (1) hour at a consistency of 1%, strained, rinsed a second time with water at approximately 20ºC, centrifuged to a fiber consistency of approximately 60%, and dried with air at room temperature to an equilibrium moisture content of approximately 8%.

The resulting individualized cross-linked cellulose-based fibers have a WRV of 37.6 and contain 3.8 mol% citric acid, calculated on a molar basis of cellulose anhydroglucose, reacted with the fibers in the form of intrafiber cross-link bonds.

The dried fibers are placed in air flow to form absorbent pads. The pads are compressed to a density of 0.20 g/cm3 using a hydraulic press. The pads are tested for absorbency, elasticity and amount of citric acid converted according to the procedures defined herein. The citric acid converted is recorded in mole percent, calculated on the basis of cellulose anhydroglucose of the dry fiber. The results are shown in Table 1 and are compared with an absorbent pad made from conventional, non-crosslinked cellulosic fibers. TABLE 1 Sample # Citric acid (mol%) converted Wet compressibility (cm³/g)

As can be seen from Table 1, the absorbent pads containing individualized citric acid cross-linked fibers (i.e., Sample 2) exhibit significantly higher drip capacities and wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads containing conventional, uncross-linked fibers (i.e., Sample 1). In addition to exhibiting improved responsiveness to wetting relative to conventional, uncross-linked fibers, the absorbent pads containing citric acid cross-linked fibers can be safely used in the vicinity of human skin.

EXAMPLE II

The individualized, cross-linked fibers of Example I are laid in air to form absorbent pads and are compressed to a density of 0.10 g/cm3 using a hydraulic press. The pads are then tested for absorbency, resilience, and structural integrity according to the wet compressibility procedure given above. The results are summarized in Table 2. TABLE 2 Sample # Wet compressibility (cm³/g)

As can be seen from Table 2, the absorbent pads containing individualized citric acid cross-linked fibers (i.e., Sample 2) -- at a dry fiber density of 0.10 g/cm3 -- exhibit significantly higher wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads containing standard, uncross-linked fibers (i.e., Sample 1). In addition to demonstrating improved responsiveness to wetting, the absorbent pads containing citric acid cross-linked fibers can also be used safely in the vicinity of human skin.

EXAMPLE III

Individualized, crosslinked fibers used in the absorbent structures of the present invention are prepared in a dry crosslinking process using 1,2,3,4-butanetetracarboxylic acid (BTCA) as the crosslinking agent. The individualized, crosslinked fibers are prepared in accordance with the process of Example I described above with the following changes: the slurry in step 2 of Example I contains 150 g of dry pulp, 1186 g of H2O, 64 g of BTCA and 4 g of sodium hydroxide. In step 5, the fibers are cured at a temperature of about 165°C for a period of about 60 minutes.

The resulting individualized, cross-linked cellulose-based fibers have a WRV of 32.9 and contain 5.2 mol% 1,2,3,4-Butanetetracarboxylic acid, calculated on a molar basis of cellulose anhydroglucose reacted with the fibers in the form of intrafiber crosslink bonds.

The dried fibers are placed in air flow to form absorbent pads. The pads are compressed to a density of 0.20 g/cm3 using a hydraulic press. The pads are tested for absorbency, elasticity and amount of 1,2,3,4-butanetetracarboxylic acid (BTCA) converted using the methods defined above. The BTCA converted is reported in mole percent calculated on the basis of cellulose anhydroglucose of the dry fiber. The results are summarized in Table 3 and are compared with an absorbent pad made from conventional, non-crosslinked cellulosic fibers. TABLE 3 Sample # Wet compressibility (cm³/g)

As can be seen from Table 3, the absorbent pads containing individualized BTCA cross-linked fibers (i.e., Sample 3) exhibit significantly higher drip capacities and wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads containing conventional uncross-linked fibers (i.e., Sample 1). In addition to exhibiting improved responsiveness to wetting, the absorbent pads containing BTCA cross-linked fibers can also be used safely in the vicinity of human skin.

EXAMPLE IV

The individualized, cross-linked fibers of Example III are laid in the air stream to form absorbent pads and compressed to a density of 0.10 g/cm3 using a hydraulic press. The pads are then tested for absorbency, resilience and structural integrity according to the wet compressibility procedure given above. The results are shown in Table 4. TABLE 4 Sample # Wet compressibility (cm³/g)

As can be seen from Table 4, the absorbent pads containing individualized BTCA cross-linked fibers (i.e., Sample 3) at a dry fiber density of 0.10 g/cm3 exhibit significantly higher wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads exhibiting the same density and containing conventional, uncross-linked fibers (i.e., Sample 1). In addition to exhibiting improved responsiveness to wetting, the absorbent pads containing BTCA cross-linked fibers can be used safely in the vicinity of human skin.

In Examples III and IV, substantially similar results are obtained when the crosslinking agent 1,2,3,4-butanetetracarboxylic acid (BTCA) is replaced in whole or in part by an equivalent amount of 1,2,3-propanetricarboxylic acid.

EXAMPLE V

Individualized, crosslinked fibers used in the absorbent structures of the present invention are prepared by a dry crosslinking process using oxydisuccinic acid (ODS) as a crosslinking agent. The individualized, crosslinked fibers are bonded to the with the following changes: the slurry in step 2 of Example I contains 140 g of dry pulp, 985 g of H₂O, 40 g of sodium salt of ODS and 10 ml of 98% sulfuric acid.

The resulting individualized, cross-linked cellulosic fibers have a WRV of 44.3 and contain 3.6 mol% oxydisuccinic acid, calculated on a molar basis of cellulose anhydroglucose, reacted with the fibers in the form of intrafiber cross-link bonds.

The dried fibers are placed in air flow to form absorbent pads. The pads are compressed to a density of 0.20 g/cm3 using a hydraulic press. The pads are tested for absorbency, elasticity and amount of oxydisuccinic acid (ODS) converted using the methods described above. The ODS converted is reported in mole percent, calculated on the basis of cellulose anhydroglucose of the dry fiber. The results are shown in Table 5 and are compared with an absorbent pad made from conventional, non-crosslinked cellulosic fibers. TABLE 5 Sample # Wet compressibility (cm³/g)

As can be seen from Table 4, the absorbent pads containing individualized ODS cross-linked fibers (i.e., Sample 4) exhibit significantly higher drip capacities and wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads containing conventional, uncross-linked fibers (i.e., Sample 1). In addition to the fact that the absorbent pads containing ODS cross-linked fibers exhibit increased responsiveness to wetting, they can also in the vicinity of human skin in a safe manner.

EXAMPLE VI

The individualized, cross-linked fibers of Example V are laid in air to form absorbent pads and are compressed to a density of 0.10 g/cm3 using a hydraulic press. The pads are then tested for absorbency, resilience, and structural integrity according to the wet compressibility procedure given above. The results are shown in Table 6. TABLE 6 Sample # Wet compressibility (cm³/g)

As can be seen from Table 6, the absorbent pads containing individualized cross-linked fibers (i.e., Sample 4) -- at a dry fiber density of 0.10 g/cm3 -- exhibit significantly higher wet compressibilities at 0.1 PSI, 1.1 PSI, and 0.1 PSIR relative to pads having the same density and containing standard, uncross-linked fibers (i.e., Sample 1). In addition to showing improved responsiveness to wetting, the absorbent pads containing ODS cross-linked fibers can also be used safely in the vicinity of human skin.

Claims (17)

1. An absorbent structure characterized by comprising individualized, crosslinked cellulosic fibers having an effective amount of a C2-C9 polycarboxylic acid crosslinking agent reacted with said fibers in an intrafiber crosslinking bond form, said C2-C9 polycarboxylic acid crosslinking agent being selected from: (i) aliphatic and alicyclic C₂-C�9 polycarboxylic acids, which are either olefinically saturated or unsaturated and have at least three carboxyl groups per molecule; and (ii) C2-C9 aliphatic and alicyclic polycarboxylic acids containing two carboxyl groups per molecule and a carbon-carbon double bond alpha, beta to one or both carboxyl groups, wherein when said carboxyl groups are separated by said double bond or when said carboxyl groups are connected to the same ring, then said carboxyl groups are in the cis configuration relative to each other; wherein in said C2-C9 polycarboxylic acid crosslinking agent one carboxyl group is separated from a second carboxyl group by either two or three carbon atoms. 2. The absorbent structure of claim 1, wherein said fibers have, calculated on a molar basis of cellulose anhydroglucose, between 0.5 mol% and 10.0 mol%, preferably between 1.5 mol% and 6.0 mol%, of crosslinking agent reacted with them in the form of intrafiber crosslinking bonds. 3. The absorbent structure of claim 2, wherein said crosslinked fibers have a water retention value of 25 to 60, preferably 28 to 50. 4. The absorbent structure according to any one of claims 1 to 3, in which said crosslinking agent is selected from citric acid, 1,2,3,4-butanetetracarboxylic acid and 1,2,3-propanetricarboxylic acid, preferably citric acid. 5. The absorbent structure according to any one of claims 1 to 3, in which said crosslinking agent is selected from oxydisuccinic acid, tartrate monosuccinic acid of the formula TEST MISSING and tartrate disuccinic acid of the formula TEXT MISSING preferably oxydisuccinic acid. 6. The absorbent structure of any one of claims 1 to 5, having a dry density and an equilibrium wet density, calculated based on dry fiber weight, said dry density being greater than said equilibrium wet density. 7. The absorbent structure of any one of claims 1 to 6, wherein said absorbent structure has a dry density of less than 0.60 g/cm3. 8. The absorbent structure of any one of claims 1 to 5, wherein said absorbent structure has a dry density between 0.05 g/cm³ and 0.15 g/cm³. 9. The absorbent structure of any one of claims 1 to 5, wherein said absorbent structure has a dry fiber density between 0.12 g/cm³ and 0.60 g/cm³ and an equilibrium wet density, calculated based on the dry fiber weight, which is less than said actual dry fiber density. 10. The absorbent structure of any one of claims 1 to 9, wherein said structure comprises between 70% and 95% individualized crosslinked fibers and between 30% and 5% uncrosslinked cellulosic fibers. 11. The absorbent structure of claim 10, wherein said non-crosslinked cellulosic fibers have a Canadian Standard freeness of less than 300. 12. The absorbent structure of any one of claims 1 to 11, further comprising a hydrogel forming material disposed on said fibers. 13. The absorbent structure of any one of claims 1 to 11, further comprising a hydrogel-forming material disposed within said absorbent structure. 14. The absorbent structure of claim 13, wherein said hydrogel-forming material is substantially homogeneously mixed within at least a portion of said absorbent structure. 15. The absorbent structure of claim 13, wherein said hydrogel-forming material is disposed on a sheet, said sheet being in contact with said fibers. 16. A disposable absorbent article comprising a topsheet, a backsheet joined to said topsheet, and an absorbent structure according to any one of claims 1 to 15 disposed between said topsheet and said backsheet. 17. The absorbent structure of any one of claims 1 to 5, wherein said absorbent structure has a basis weight of less than 800 g/m² and a dry density of less than 0.60 g/cm³.